The Astrophysical Journal, 683: L59–L62, 2008 August 10 ൴ ᭧ 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A.

THE SPIN-ORBIT ALIGNMENT OF THE HD 17156 TRANSITING ECCENTRIC PLANETARY SYSTEM1,2 William D. Cochran, Seth Redfield,3 Michael Endl, and Anita L. Cochran McDonald Observatory and Department of Astronomy, The University of Texas at Austin, Austin, TX 78712; [email protected], sredfi[email protected], [email protected], [email protected] Received 2008 May 27; accepted 2008 June 25; published 2008 July 18

ABSTRACT We present high-precision observations of HD 17156 during a of its eccentric Jovian planet. In these data, we detect the Rossiter-McLaughlin effect, which is an apparent perturbation in the velocity of the due to the progressive occultation of part of the rotating stellar photosphere by the transiting planet. 25Њ misalignment ofעThis system had previously been reported by Narita et al. in 2008 to exhibit al p 62Њ the projected planetary orbital axis and the stellar rotation axis. We model our data, along with the Narita et al. 9.3Њ for the combined data set. We thus conclude that the planetary orbital axis isעdata, and obtainl p 9.4Њ actually very well aligned with the stellar rotation axis. Subject headings: planetary systems — : individual (HD 17156) Online material: color figure

1. INTRODUCTION (Holman et al. 1997; Wu & Murray 2003) can result in large eccentricities and significant inclination of planetary orbital Transiting extrasolar planets allow us to perform critical tests planes from the plane of the stellar equator (which is presum- of models of planetary system formation and evolution. In our ably close to the plane of the inner portion of the protoplanetary but not solar system, all of the planets orbit approximately, disk). While the shortest period transiting systems have prob- exactly , in the solar equatorial plane. Beck & Giles (2005) find ably been tidally circularized, longer period systems can easily that the angle between the plane of the ecliptic and the solar Њ retain large eccentricities over the main-sequence lifetime of עЊ equator is7.155 0.002 . The near coplanarity of the plan- the parent star. A planet that has undergone significant gravi- etary orbits in our solar system has influenced models of plan- tational scattering or Kozai excitation would not necessarily etary system origin from the time of Kant (1755) and Laplace retain a low inclination relative to the stellar equator. Thus, the (1796) to essentially all modern models (e.g., Lissauer 1995; report of a possible spin-orbit misalignment of the HD 17156b Pollack et al. 1996; Boss 2000). Given the level of misalign- transiting planet by Narita et al. (2008) is extremely interesting ment in our solar system and the observation of warps in debris and calls for further detailed investigation. In this Letter, we b disks around nearby stars such as Pic (e.g., Burrows et al. report our own spectroscopic observations of a transit of HD 1995; Mouillet et al. 1997; Heap et al. 2000), it is obvious that 17156 by its planet, using two telescopes at McDonald there are common processes that give rise to some small level Observatory. of spin-orbit misalignment in planetary systems. The degree of misalignment in real planetary systems must depend on the 2. OBSERVATIONS initial conditions (the initial asymmetries of the collapsing cloud), the physics of disk formation and evolution, and the We observed the transit of HD 17156 by its hot-Jupiter com- physics of stellar and angular momentum loss during the panion on the night of 2007 December 25 UT, using both the T Tauri phase. Most transiting planets have periods of just a 2.7 m Harlan J. Smith Telescope (HJST) and the Hobby-Eberly few days and orbits of low eccentricity. Significant exceptions Telescope (HET) at McDonald Observatory. The HJST obser- are HD 147506b (HAT-P-2b) (Bakos et al. 2007), with a 5.6 vations used the 2d coude´ spectrograph (Tull et al. 1995) in and eccentricity of 0.52, HD 17156 (Fischer its “F3” mode, which gives a spectral resolving power of et al. 2007; Barbieri et al. 2007), which has the longest orbital R p l/dl p 60,000. This mode is referred to as “cs23.” A period (21 days) and the largest eccentricity (0.67) of all of temperature-stabilized I2 gas absorption cell is used to impose the transiting , and XO-3b, a massive (13.25MJup ) the velocity metric for precise radial velocity measurements of planet that has an eccentricity of 0.26 in spite of its short orbital the stellar spectrum. Details of the 2.7 m cs23 observing and period of 3.192 days (Johns-Krull et al. 2008). Scenarios for data reduction procedures are given by Endl et al. (2004, 2006). the formation of short-period planetary systems do not nec- Velocity observations were started before the expected begin- essarily deliver those planets to their present semimajor axes ning of the transit and were continued until after the expected with low eccentricity. Type II migration can result in planets end of the transit. At total of 18 spectra, each 15 minutes in with moderate eccentricities (Sari & Goldreich 2004), while length, were obtained. Table 1 gives the relative radial velocities dynamical interactions among newly formed planets and plan- for the HJST observations of HD 17156. etesimals (Juric´ & Tremaine 2008) or the Kozai mechanism HET observations were made on the same night (2007 De- cember 25 UT) using the High Resolution Spectrograph (HRS) 1 Based on observations obtained with the Hobby-Eberly Telescope, which (Tull 1998) in itsR p 60,000 mode. Due to its fixed-zenith- is a joint project of the University of Texas at Austin, the Pennsylvania State distance design, we were able to observe HD 17156 only from University, Stanford University, Ludwig Maximilians Universita¨t Mu¨nchen, shortly before the beginning of the transit to just past midtransit. and Georg August Universita¨t Go¨ttingen. 2 This Letter includes data taken at the McDonald Observatory of the Uni- The observations were planned to obtain as many 600 s ex- versity of Texas at Austin. posures of HD 17156 as possible during the 2.1 hr track length. 3 Hubble Fellow. The 13th target exposure on the HET was terminated after L59 L60 COCHRAN ET AL. Vol. 683

TABLE 1 TABLE 2 McDonald Observatory 2.7mHJST Hobby-Eberly Telescope HRS Relative Relative Velocities for HD 17156 Velocities for HD 17156

BJD Velocity j BJD Velocity j (Ϫ2,400,000) (m sϪ1) (m sϪ1) (Ϫ2,400,000) (m sϪ1) (m sϪ1) 54,459.604759 ...... Ϫ7898.41 9.14 54,459.605807 ...... 18.55 8.06 54,459.616367 ...... Ϫ7905.69 9.79 54,459.621927 ...... 10.35 8.01 54,459.627985 ...... Ϫ7936.46 9.57 54,459.629827 ...... 5.63 7.13 54,459.639520 ...... Ϫ7912.68 10.41 54,459.637718 ...... 0.74 8.11 54,459.651060 ...... Ϫ7913.60 8.61 54,459.645611 ...... 19.18 6.94 54,459.663985 ...... Ϫ7921.94 12.61 54,459.653504 ...... 11.45 6.32 54,459.675520 ...... Ϫ7916.51 11.56 54,459.661400 ...... 4.77 7.46 54,459.687056 ...... Ϫ7936.95 10.14 54,459.669293 ...... 4.65 7.34 54,459.698591 ...... Ϫ7955.00 11.86 54,459.677194 ...... Ϫ4.16 7.34 54,459.710170 ...... Ϫ7960.05 12.44 54,459.685096 ...... Ϫ7.11 6.32 54,459.721705 ...... Ϫ7985.06 10.98 54,459.692989 ...... Ϫ9.91 7.68 54,459.733240 ...... Ϫ7964.48 12.75 54,459.700887 ...... Ϫ18.44 7.99 54,459.744778 ...... Ϫ8003.38 10.36 54,459.707944 ...... Ϫ35.93 9.25 54,459.757932 ...... Ϫ7977.30 11.57 54,459.769469 ...... Ϫ7979.22 11.89 54,459.781004 ...... Ϫ7988.41 12.40 We analyzed our data using a model that is a hybrid of these 54,459.792539 ...... Ϫ7990.68 10.82 methods. We started by adopting the HD 17156b system pa- Ϫ 54,459.804150 ...... 8010.27 10.81 rameters from Narita et al. (2008). We then computed the orbit of the planet around the star, as we would view it from Earth. 455 s when the fiber-instrument-feed reached the end of track. This gave us the apparent offset of the planet from the center Details of the instrument configuration and the data reduction of the star as a function of time through the transit. We divided and analysis procedures are given by Cochran et al. (2004, the stellar disk into a400 # 400 grid of cells, in the manner 2007). Table 2 gives the relative radial velocities for the HET of Queloz et al. (2000), Snellen (2004), or Winn et al. (2005). observations of HD 17156. For each photospheric cell, we computed a specific intensity For both the HJST and HET data, observation times and using the nonlinear four-parameter limb-darkening law of velocities have been corrected to the solar system barycenter. Claret (2000). Each photospheric cell is also assigned a radial The uncertainty j for each velocity in the table is an internal velocity due to both the stellar orbital motion and the stellar error computed from the variance about the mean of the ve- rotation, with the stellarv sin i as a model parameter. For each locities from each of the ∼2A˚ small chunks into which the time step during the transit, from first contact to fourth contact, spectrum is divided for the velocity computation. Thus, it rep- we compute which stellar photospheric cells are blocked by resents the relative uncertainty of one velocity measurement the transiting planet. We then integrate the unblocked Doppler- with respect to the others for that instrument, based on the shifted and intensity-weighted stellar photospheric cells to com- quality and observing conditions of the spectrum. This uncer- pute both the RM radial velocity perturbation during the transit tainty does not include other intrinsic stellar sources of uncer- and the transit photometric light curve. tainty, nor any unidentified sources of systematic errors. The We fully recognize the limitations and approximations in- two different spectrographs have independent arbitrary velocity herent in this modeling procedure. First, there is no a priori zero points, and thus there is some constant offset velocity reason to assume that limb darkening should follow any par- (determined below and denoted as g) between the data sets ticular law. Also, the limb darkening in photospheric absorption presented in Tables 1 and 2. lines is quite different from the limb darkening in the contin- uum. While this appears to be taken into account by Claret 3. ROSSITER-MCLAUGHLIN EFFECT MODEL (2000), the limb-darkening parameterization is based on model atmospheres rather than on real stars. More importantly, in our A variety of different types of models have been used by model we compute the specific-intensity–weighted apparent others to analyze observations of the Rossiter-McLaughlin Doppler shift of the visible portion of the photosphere. On the (RM) (Rossiter 1924; McLaughlin 1924) effect for transiting other hand, our spectra record transit-perturbed stellar absorp- planets. Queloz et al. (2000) divided a model stellar photo- tion-line profile shapes from which we measure an apparent sphere into a large number of cells and then used a “Gaussian Doppler shift using a computer code that assumes an unper- shape cross-correlation model” with a linear limb-darkening turbed line-profile shape. In future improvements to our RM law to compute the radial velocity anomaly. Ohta et al. (2005) model, we will attempt to improve several of these limitations. developed analytic expressions for the apparent radial velocity perturbation during a transit, in several different approxima- 4. DATA ANALYSIS tions. Gime´nez (2006) developed another set of analytic ex- pressions for the RM effect that utilize a more generalized We used the model described in§3toanalyzesimulta- higher order limb-darkening expression. A more elaborate tech- neously the data sets from the HJST cs23, the HET HRS, and nique was developed by Winn et al. (2005), who first computed the observations published by Narita et al. (2008), which we an approximation to the disk-integrated stellar spectrum. They will refer to as the “OAO/HIDES” data set. Since each data then computed a Doppler-shifted and intensity-scaled spectrum set has its own independent velocity zero point, we allowed of the portion of the disk that would be blocked by the transiting the systemic velocity of each data set to be an independent free planet and subtracted this from their disk-integrated spectrum. parameter in the analysis. The values of the fixed parameters This spectrum was then multiplied by their high-resolution io- for the analysis are given in Table 3. The planetary orbital dine spectrum, and the result was processed through their radial elements were taken from Irwin et al. (2008). These elements velocity code to compute model velocities in the same manner are essentially indistinguishable from the single-planet fit of as the observed data. Short et al. (2008). We note that the conclusions of this work No. 1, 2008 SPIN-ORBIT ALIGNMENT OF HD 17156b L61

TABLE 3 Assumed System Parameters

Parameter Value Source

MM∗ (, ) ...... 1.2 Fischer et al. 2007 RR∗ (, ) ...... 1.47 Fischer et al. 2007 Claret a1...... 0.5346 Claret 2000 a2 ...... 0.1041 Claret 2000 a3 ...... 0.4189 Claret 2000 a4 ...... Ϫ0.2584 Claret 2000

RRp (Jup )...... 1.01 Irwin et al. 2008 P (days) ...... 21.21691 Irwin et al. 2008

Tp (HJD) ...... 2,453,738.605 Irwin et al. 2008 K (m sϪ1) ...... 273.8 Irwin et al. 2008 e ...... 0.670 Irwin et al. 2008 q (deg) ...... 121.3 Irwin et al. 2008 depend on the particular values of these parameters we adopt. If any of these parameters turn out to be in significant error, those errors will propagate through this analysis. In modeling the data, we allowed the orbital plane inclination i, the projected angle l between the planetary orbital axis and the stellar rotation axis, the projected stellar rotational velocity v sin i, as well as the systemic velocity of each separate data 2 set to be free parameters. We then minimized the x of the Fig. 1.—The fit of our Rossiter-McLaughlin effect model (solid black line) model fit to the data. to the observational data sets. The HJST cs23 data are shown as the (red) 9.3Њ at crosses, the HET HRS data are (blue) solid squares, and the OAO/HIDES dataעFor the combined data sets, we obtainedl p 9.4Њ 0.4Њ . This indi- are (green) open triangles. The dashed black line is the orbital velocity of theעkm sϪ1 andi p 85.9Њ 1.1 ע v sin i p 6.3 star in the absence of any Rossiter-McLaughlin velocity perturbation. [See the cates that the planet is orbiting near the stellar equatorial plane, electronic edition of the Journal for a color version of this figure.] to within the uncertainty of our determination. Our derived v sin i is somewhat larger than the value of 4.7 m sϪ1 found Ϫ1 Doppler shift of the portion of the stellar photosphere eclipsed by Narita et al. (2008) and the 2.6 km s given by Fischer et by the planet. Thev sin i p 0 model gave x 2 p 71.56 al. (2007). Our inclination is within 1 j of the Narita et al. (x 222p 1.43 ) as opposed tox p 40.53 (x p 0.83 ) for the Њ rr value of 85.65 . However, our value for l from the combined best-fitting RM model for the combined data set. Thus, we data differs significantly from that of Narita et al. (2008), who Њ conclude that the RM effect was indeed convincingly detected עp Њ foundl 62 25 . in spite of the apparent noise in the data. Examining thex 2 of In order to understand the reason for this difference, we then each individual data set in the full model and the v sin i p 0 modeled each of the three data sets separately. The derived l model showed that even in the noisiest case of the OAO/HIDES and i for the combined fit, as well as for each individual data data, the RM effect reduced the totalx 2 by 13.1. set, are given in Table 4. We also givegc , the systemic velocity Due to the symmetric signature of the RM effect in the for each data set in the combined fit, as well asgi , the systemic l ≈ 0 case, it is critically important to sample the entire transit. velocity for each individual data set fit. From the OAO/HIDES Њ p This is evidenced by the fit to the HET/HRS data alone, in עp Њ data set alone, we computedl 59.4 19.7 at i which despite the high quality of the data, only the first half 0.8Њ. If we fix the inclination at thei p 85.65Њ valueע86.2Њ -Њ of the transit was observable, and therefore erroneous param עp Њ of Narita et al. (2008), we getl 61.4 28.5 . The sur- eters are derived. prisingly excellent agreement of all of these values for the In order to successfully calibrate the baseline radial velocity OAO/HIDES data set thus validates the modeling process of variation, observations well before and after the transit are Narita et al. We note that the results from the HJST data alone required. If the radial velocity can be well calibrated by ob- are in very good agreement with the combined results. Due to servations before and after transit, the influence of uncertainty the design limitations of the HET, the HET/HRS data covered in the zero-point velocity offset (g) will be negligible on the only the first half of the transit. Thus, the code could trade off RM effect parameters. l versus the systemic velocity for the data set and derived a 25.2Њ for the HET dataעslightly lowerxl2 withp Ϫ32.4Њ 5. DISCUSSION alone. The model fit to the data is shown in Figure 1. We also We have reanalyzed the data of Narita et al. (2008) along computed a model with no RM effect by settingv sin i p 0 . with our own independent observations of the spectroscopic This removes the stellar rotation, and thus there is no apparent transit of the eccentric HD 17156b, and we find that

TABLE 4 Rossiter-McLaughlin Model Results

l i v sin i gc gi Data Set (deg) (deg) (km sϪ1) x2 dof (m sϪ1) (m sϪ1) 49 40.53 1.1 ע 6.3 0.4 ע 85.9 9.3 ע Combined ...... 9.4 Ϫ7853.0 Ϫ7854.0 14 13.19 2.1 ע 7.1 0.6 ע 86.1 15.6 ע HJST cs23 ...... 4.5 65.6 73.3 9 3.71 1.5 ע 4.8 0.5 ע 86.3 25.2 ע HET HRS ...... Ϫ32.4 146.5 142.1 21 14.25 1.9 ע 8.6 0.8 ע 86.2 19.7 ע OAO/HIDES ...... 59.4 L62 COCHRAN ET AL. Vol. 683

,9.3Њ. We conclude that this exoplanetary system would be no induced change in the . Thusעl p 9.4Њ is similar to almost all of the other short-period transiting exo- it appears that the dynamical process that placed HD 17156b planetary systems studied using the RM effect so far, in that into a short-period highly eccentric orbit did not significantly it shows that the projected planetary orbital axis appears to be affect the orbital inclination of the system with respect to the aligned with the projected stellar rotation axis, to within our stellar equatorial plane. measurement precision. The only remaining notable exception is XO-3b, which was reported by Hebrard et al. (2008) to show We are extremely grateful for receiving Director’s Discre- . 15Њעa very significant misalignment ofl p 70Њ tionary Time on the HET on very short notice in order to The HD 17156b system is extremely interesting because it obtain the data presented here. S. R. would like to acknowl- has a very high eccentricity (e p 0.67 ) for such a short orbital edge support provided by NASA through Hubble Fellowship period of 21.2 days. Most other transiting systems have much grant HST-HF-01190.01 awarded by STScI, which is operated shorter orbital periods and eccentricities near zero. Many of by AURA Inc., for NASA, under contract NAS 5-26555. This the interesting ways of getting a planet into this type of orbit material is based on work supported by NASA under grant might well result in the possibility of an orbital plane signif- NNG05G107G issued through the TPF Foundation Science icantly inclined to the stellar equatorial plane. Dynamical in- Program and under Cooperative Agreement NNA06CA98A teractions with another nearby planet could result in ejection issued through the Kepler Program. The Hobby-Eberly Tele- of the other body and a large orbital inclination of the surviving scope (HET) is a joint project of the University of Texas at body. Austin, the Pennsylvania State University, Stanford Univer- The presence of a third planet in the system, as suggested sity, Ludwig Maximilians Universita¨t Mu¨nchen, and Georg by Short et al. (2008), would have some effect on the orbital August Universita¨t Go¨ttingen. The HET is named in honor elements of HD 17156b. However, as Short et al. (2008) dis- of its principal benefactors, William P. Hobby and Robert E. cussed, the primary effects are a small oscillation of the ec- Eberly. centricity and a secular advance of the argument of periastron. As long as the two planets are approximately coplanar, there Facilities: HET, McD:2.7m.

REFERENCES

Bakos, G. A´ ., et al. 2007, ApJ, 670, 826 Kant, I. 1755, General History of Nature and Theory of the Heavens (Ko¨n- Barbieri, M., et al. 2007, A&A, 476, L13 ingsberg: Perersen) Beck, J. G., & Giles, P. 2005, ApJ, 621, L153 Laplace, P. S. 1796, Exposition du syste`me du monde (Paris: Cercle-Social) Boss, A. P. 2000, ApJ, 536, L101 Lissauer, J. J. 1995, Icarus, 114, 217 Burrows, J. C., Krist, J. E., Stapelfeldt, K. R., & WFPC2 Investigation Def- McLaughlin, D. B. 1924, ApJ, 60, 22 inition Team. 1995, BAAS, 27, 1329 Mouillet, D., Larwood, J. D., Papaloizou, J. C. B., & Lagrange, A. M. 1997, Claret, A. 2000, A&A, 363, 1081 MNRAS, 292, 896 Cochran, W. D., Endl, M., Wittenmyer, R. A., & Bean, J. L. 2007, ApJ, 665, Narita, N., Sato, B., Ohshima, O., & Winn, J. N. 2008, PASJ, 60, L1 1407 Ohta, Y., Taruya, A., & Suto, Y. 2005, ApJ, 622, 1118 Cochran, W. D., et al. 2004, ApJ, 611, L133 Pollack, J. B., Hubickyj, O., Bodenheimer, P., Lissauer, J. J., Podolak, M., & Endl, M., Cochran, W. D., Wittenmyer, R. A., & Hatzes, A. P. 2006, AJ, 131, Greenzweig, Y. 1996, Icarus, 124, 62 3131 Queloz, D., Eggenberger, A., Mayor, M., Perrier, C., Beuzit, J. L., Naef, D., Endl, M., Hatzes, A. P., Cochran, W. D., McArthur, B., Prieto, C. A., Paulson, Sivan, J. P., & Udry, S. 2000, A&A, 359, L13 D. B., Guenther, E., & Bedalov, A. 2004, ApJ, 611, 1121 Rossiter, R. A. 1924, ApJ, 60, 15 Fischer, D. A., et al. 2007, ApJ, 669, 1336 Sari, R., & Goldreich, P. 2004, ApJ, 606, L77 Gime´nez, A. 2006, ApJ, 650, 408 Short, D., Welsh, W. F., Orosz, J. A., & Windmiller, G. 2008, preprint (arXiv: Heap, S. R., Lindler, D. J., Lanz, T. M., Cornett, R. H., Hubeny, I., Maran, 0803.2935) S. P., & Woodgate, B. 2000, ApJ, 539, 435 Snellen, I. A. G. 2004, MNRAS, 353, L1 Hebrard, G., et al. 2008, A&A, in press (arXiv:0806.0719v1) Tull, R. G. 1998, Proc. SPIE, 3355, 387 Holman, M., Touma, J., & Tremaine, S. 1997, Nature, 386, 254 Tull, R. G., MacQueen, P. J., Sneden, C., & Lambert, D. L. 1995, PASP, 107, Irwin, J., et al. 2008, ApJ, 681, 636 251 Johns-Krull, C., et al. 2008, ApJ, 677, 657 Winn, J. N., et al. 2005, ApJ, 631, 1215 Juric´, M., & Tremaine, S. 2008, ApJ, in press (astro-ph/0703160) Wu, Y., & Murray, N. 2003, ApJ, 589, 605